Quickbooks stores its data in password-protected .QBB files. Are these files encrypted with this password? If someone gets .QBB file, do they need the password (either by knowing it or by brute forcing it) to access the accounting records?
You didn't state your OS, QB version, or reason for wanting to know why, but I assume you're either
(ignoring the other option of wanting to open someone else's backup file).
If secure storage is the issue, you can either use the Windows Encrypting File System, or go to the next level and use the excellent open source TrueCrypt. Either will give you on-the-fly transparent secure storage, although the former will allow access to anyone with your Windows user password.
If secure transfer is the issue, you could again use TrueCrypt to encrypt the file before sending it, or else you could use any of the zip / compression utilities that provide encryption. Personally I prefer 7-zip for its open source nature, but most compression tools include similar functionality.